Abstract

The local environment of manganese atoms in LaMnO{sub 3} under pressure up to 15.3 GPa has been studied by x-ray absorption spectroscopy. For pressures below 8 GPa, no change is detected within the MnO{sub 6} octahedra. Above this pressure a continuous reduction of the long Mn-O distance takes place, however, the octahedral distortion persists over the whole pressure range. At 15.3 GPa the average Jahn-Teller splitting of the distances is reduced by about one-third, indicating that a total removal of the local Jahn-Teller distortion would occur only for pressures around 30 GPa, where metallization is reported to take place. A hysteresis in the long distance reduction is observed down to ambient pressure, suggesting the coexistence of MnO{sub 6} distorted and undistorted units.

The b-axis oriented orthorhombic YMnO{sub 3} (o-YMnO{sub 3}) epitaxial films on a YAlO{sub 3} (010) substrate were fabricated with pulsed-laser deposition. The anisotropic orbital occupation and Jahn-Teller (JT) distortion of an o-YMnO{sub 3} film were investigated with polarization-dependent x-ray absorption spectra and configuration-interaction multiplet-cluster calculations. A significant energy difference, ∼3.8 eV, for the main white line along E//b and E//a in polarization-dependent Mn K-edge spectra of o-YMnO{sub 3} indicates an extraordinary JT distortion and significant anisotropic Mn–O bonding within the ab plane in the o-YMnO{sub 3} film. Most importantly, although the orbital occupation of 3d electrons in o-YMnO{sub 3} filmsmore » is almost the same as that in single crystalline o-DyMnO{sub 3}, the JT distortion of o-YMnO{sub 3} films is larger than that of single crystalline o-DyMnO{sub 3}, deduced from the multiplet calculations. We speculate that this JT distortion predominantly contributes to the origin of the cycloidal spin deformation in bulk o-YMnO{sub 3}, because of a suppressed nearest-neighbor superexchange interaction and an enhanced next-nearest-neighbor superexchange interaction. These complementary results provide insight into the origin of the E-type magnetic configuration of o-YMnO{sub 3}.« less

A synchrotron x-ray diffraction study of the crystallographic structure of PrO{sub 2} in the Jahn-Teller distorted phase is reported. The distortion of the oxygen sublattice, which was previously ambiguous, is shown to be a chiral structure in which neighboring oxygen chains have opposite chiralities. A temperature dependent study of the magnetic excitation spectrum, probed by neutron inelastic scattering, is also reported. Changes in the energies and relative intensities of the crystal field transitions provide an insight into the interplay between the static and dynamic Jahn-Teller effects.

Structural changes accompanying the electrochemical Li deintercalation of Li{sub 1{minus}x}NiO{sub 2} and Li{sub 1{minus}x}CoO{sub 2} were studied by the transmission X-ray absorption fine structure (XAFS) technique using an in situ X-ray cell of original design. The results revealed that the Jahn-Teller distortion of the Ni-O octahedra found in LiNiO{sub 2} decreased as the Li ion was removed from the cathode. The observed X-ray absorption near-edge structures (XANES) of Li{sub 1{minus}x}NiO{sub 2} and Li{sub 1{minus}x}CoO{sub 2} as a function of x are consistent with the phase transitions found by powder-diffraction studies. The Ni-O, Co-O, and Ni-Ni interatomic distances decreased almost linearlymore » as the Li content decreased to x = 0.8, while the Co-Co distance slightly decreased to x = 0.5 and increased up to x = 0.8. The local distortion of NiO{sub 6} octahedra also decrease with increasing y in a Li(Ni{sub 1{minus}y}, Co{sub y}){sub 2} solid solution.« less

We have investigated the local structure around the Mn atom on highly diluted manganites with formal Mn{sup 3+} and Mn{sup 3.5+} ions by x-ray absorption fluorescence spectra. Both x-ray absorption near-edge structure and extended x-ray absorption fine structure were recorded for LaMn{sub 1-x}Ga{sub x}O{sub 3} (x=0.8 and 0.9) and La{sub 0.9}Ca{sub 0.1}Mn{sub 0.2}Sc{sub 0.8}O{sub 3} compounds at room temperature. The local structure around the Mn atom in the gallium samples consist of a regular octahedron of oxygen atoms contrasting with the tetragonal Jahn-Teller distortion expected for a formal Mn{sup 3+} ion. This result suggests that the ordered tetragonal distortions inmore » LaMnO{sub 3} are not originated by a single ion mechanism like the Jahn-Teller one.« less